The telecommunication transport network has been evolved from a time division multiplexing (TDM) transport network, for example, synchronous digital hierarchy (SDH)/synchronous optical network (SONET) to a packet switching (PS) transport network. In order to ensure the network stability and service continuity, a progressive evolution manner is usually adopted, so that the two transport networks coexist and are converged for quite a long time.
The two transport networks have their respective clock transmission manners. The clock transmission of the TDM transport network is based on the SDH protocol. Referring to FIG. 1, the clock transmission manner for the SDH network is shown. It may be known that the equipment in the SDH network acquires a clock from a primary reference clock (PRC), then transfers the clock in a physical layer, and transfers a synchronization status message (SSM) representing a clock quality class through a multiplex section overhead of a multiplex section based on the SDH protocol. The SSM is defined as follows.
TABLE 1Quality ClassValue (hex)Please AddClockQL-PRC0X10  <1*10−11Class-1 ClockQL-SSU-A0X1001.6*10−8Rubidium ClockClass-2 ClockQL-SSU-B10004.6ppmLocal ClockClass-2 ClockQL-SEC1011<4.6ppmLocal ClockQL-DNU1111
The PS transport network has two clock transmission manners.
The first one is a synchronous Ethernet clock transmission.
The synchronous Ethernet clock transmission is also based on the SDH protocol. Referring to FIG. 2, the equipment in the synchronous Ethernet acquires a clock from the PRC, then transfers the clock in the physical layer, and transfers the SSM representing the clock quality class through an Operation, Administration, and Maintenance (OAM) packet based on the SDH protocol.
The second one is an IEEE 1588 PTP clock transmission.
The IEEE 1588 PTP clock transmission is based on a precision time protocol (PTP). Referring to FIG. 3, the time and the time quality class are transferred through an IEEE 1588 PTP packet based on the PTP. For example, the timeSource information representing an initiation point of a reference time and the information of the PTP Clock Class information representing the time quality class are transmitted through an announce packet, and a precise clock is transmitted through an Sync (synchronous) packet. The time difference information is transmitted through delay-req (delay request) and delay-resp (delay response) packets. The head formats of the above packets are the same. According to the timeSource, the types of the clocks may be identified. According to the information of the PTP Clock Class, the clock frequency information may be computed. According to the time difference information, the current system time may be adjusted.
As seen from the above three clock transmission manners of the two transport networks, the following conclusion is reached.
As for the clock transmission manners of both the SDH network and the synchronous Ethernet, the clock is transferred via the physical layer, and the SSM representing the clock quality class is transferred based on the SDH protocol. Thus, when the SDH network and the synchronous Ethernet are converged, the clock transmission can also be well united. As for the IEEE 1588 PTP network, the time precision class is determined according to the timeSource information and the information of the PTP Clock Class transmitted based on the PTP, and the information has nothing to do with the clock, which is significantly different from the SDH network in which the clock quality class is determined according to the SSM transmitted based on the SDH protocol. Therefore, the equipment that supports merely one of the protocols cannot compare the qualities of the clocks transferred based on different protocols, so that the clock with the higher class cannot be selected. For example, referring to FIG. 4, an upper layer network is the IEEE 1588 PTP network, and the TDM network is connected subsequently. The IEEE 1588 PTP network can provide a recovered class-1 clock to the TDM network. After receiving the clock provided by the IEEE 1588 PTP network, the equipment in the TDM network fails to determine whether the quality class of the clock provided by the IEEE 1588 PTP network is higher than that of the class-2 clock provided by a building integrated timing supply system (BITS), since no SSM representing the clock quality class is carried. Therefore, the equipment only selects the class-2 clock provided by the BITS, instead of the class-1 clock provided by the IEEE 1588 PTP network, that is to say, the equipment in the TDM network selects the clock with a lower class instead of the one with a higher class as a clock source.